Many products are packaged in trays which are overwrapped with a clear plastic film. The clarity of the film adds to the market appeal and ultimate commercial success of the product. It is also important for a package to be wrapped so that the plastic film is smooth in appearance, rather than wrinkled, in order to optimize the consumer appeal of the product. These basic sales concepts apply to packages wrapped in either heat shrinkable or stretch-wrap film, either of which may be employed for wrapping of product-filled trays.
A packaging apparatus which is representative of the state of the art is taught in U.S. Pat. No. 4,841,715 to Suga. The '715 patent discloses an apparatus having a pair of chains which run along opposite sides of a path of transport of the article to be wrapped. The chains carry a number of grippers, or clamps, which are adapted to firmly grasp each edge of a wrapping film. The grippers operate to place the film over the article being wrapped and pull the edges of the film downwardly in the widthwise direction so as to minimize wrinkles. The width between the pair of chains is adjustable according to the basic size of the film and whether the film is of the heat shrinkable or stretch-wrap type. The teachings of the '715 patent are incorporated herein by reference.
A significant limitation of the '715 patent apparatus to which the present invention is directed relates to the amount of stretch which can be placed on the film. In the '715 patent apparatus, the grippers used comprise two parallel horizontal plates biased so as to firmly grip a wrapping film when the film is placed between the plates and the plates closed. The film is brought downwardly upon the article being wrapped and is pulled further down as the film wraps around the article. In practice, the downward pull of the grippers on the film results in an upward pull of the film upon the grippers, which upward pull tends to either tear the film or open the grippers and release the film. Therefore, the amount of stretch which may be placed on the film is limited by the holding strength of the grippers and the tensile strength of the film.
It is useful to be able to stretch a wrapping film to a greater degree than is possible with the '715 patent apparatus. One reason for greater stretch is to be able to remove wrinkles as much as possible during the wrapping process. Another reason is to be able to use and stretch heavier gauge films, which heavier films will better protect the article being wrapped. An additional reason is to be able to stretch the film widthwise and thereby obtain an effectively wider film for the cost of a narrower film.
Therefore, the foregoing define objectives of the invention as will be apparent in the disclosure that follows.